High power electrical converters are used for converting voltages of different amplitudes and frequencies (such as AC and DC voltages) into one another. In such converters, semiconductor switches are switched according to specific patterns determined by a controller to achieve the voltage conversion.
One possibility to control such a converter are pre-computed optimized pulse patterns (OPP), which may be used to shape the spectrum of the currents and voltages of the converter. Traditional OPP-based methods rely on an external control loop to provide a modulation index reference that is used to read offline computed switching angles. These switching angles are then directly applied to the semiconductor switches.
Such a method may have several drawbacks. For example, the dynamic performance may be poor because of the slow modulation index control loop. Also, the OPP angles may have to change slowly when changing the modulation index, i.e., almost continuous. The necessity for smooth OPP angles greatly limits the achievable current and voltage spectrums over the operating range.
Therefore, in EP 2 469 692 A1, optimized pulse patterns are modified based on model predictive control. Pre-computed OPP switching angles are modified online in order to follow a flux trajectory. This model predictive control method may solve the two issues mentioned above and may be well suited for machine-side applications.
However, for grid side applications, it may be preferable to directly control the current. In a flux-based method for the grid side, the current reference may have to be transformed into a voltage reference and then integrated over time to generate a flux reference. In such a way, generating the voltage reference may require good knowledge of the grid voltage and impedance, as well as parameters of a transformer interconnected between the grid and the converter.
Another issue with flux-based control on the grid side may be that generating a flux reference by integrating the voltage reference can cause long lasting off-centered flux trajectories when sudden changes in the voltage amplitude occur. This is the case for grid voltage dips, for instance.
EP 0 504 449 A1 relates to a control method for an electrical converter, in which OPPs are modified based on a voltage reference.
Lacynsky T. et al, “Modulation Error Control for Medium Voltage Drives with LC-Filters and Synchronous Optimal Pulse Width Modulation, INDUSTRY APPLICATIONS SOCIETY ANNUAL MEETING, 2008. IAS '08. IEEE, IEEE, PISCATAWAY, N.J., USA, Oct. 5, 2008 refers to a simulation and describes that precalculated optimized pulse patterns may be optimized by eliminating undesired harmonics (chapter “Definition and Calculation of Modulation Error” on page 3). A modulation error may be corrected, either based on an analytical solution of the optimization problem (chapter “Modulation Error Control by the LQR Controller” on page 4) or directly by minimizing an energy function (chapter “Modulation Error control by the Ljapunow Controller” on page 4).